1. Field of the Invention
The present invention relates to closures and plugs, more specifically to a nozzle dam which is segmented and contoured for high strength-to-weight ratio, for portability and for rapid installation in a confined space. One application, for example, is sealing a nozzle from within a nuclear steam generating vessel.
During maintenance of a nuclear steam generation system, it is necessary for an operator to enter the steam generator vessel to seal a nozzle against reactor water which is under pressure to seek an equilibrium level by entering the vessel.
The operator must enter the vessel by way of a manway, an opening that is smaller in diameter than the opening of the nozzle to be sealed. Consequently the dam must be temporarily reduced in size by some means so that it can be passed through the manway.
The interior environment of the vessel is radioactive and so confined that usually only a single operator at a time can operate there to manipulate and fasten the dam in place, maximum allowable exposure for a single individual being limited to a few minutes.
2. Description of the Prior Art
A nozzle plug that is capable of being used to seal a nuclear steam generator nozzle is described in U.S. Pat. No. 4,482,076, patented Nov. 13, 1984 by Timothy H. Wentzell.
The plug consists of a sectioned base plate, each section being reinforced by upstanding, parallelly extending side wall members or ribs with lightening holes, fixedly attached to the planar base portion of the base plate.
The sections are joined together at the ribs to make the base plate. They are fastened together through adjacent ribs by a combination of pins in offset slots and quick-disconnect pin fasteners.
For added strength and rigidity, the ribs meet in a series of open boxes as seen from the top of the plate. Alternatively, cross pieces are fastened across the ribs and bolted between the ribs, to the assembled disk.
The plate is installed in the nozzle opening, generally below the frustoconical throat of the nozzle. It is held in place by quick-disconnect fasteners having pins extending radially through bosses in an annular peripheral flange on the plate, and into openings in the side wall surfaces of the nozzle below the frustoconical throat of the nozzle.
A pair of inflatable beads, formed as a double rim around the circumference of the plate, are sealingly joined with a resilient member that covers the surface of the plate that faces into the nozzle. The beads are filled with air from a pump to form a seal between the plate and the nozzle wall. Fluid pressure against the outer surface of the resilient member tends to urge one inflated rim against the other which is then urged into engagement with the circumferential rim portions of the plate.
In an alternative embodiment, two plates are fastened as described above, within the nozzle below the frustoconical opening. Each plate has only one inflatable bead rim, and in each, the base plate is a concave wall of generally uniform thickness, to which the ribs with lightening holes are attached.
To install the dam, the operator assembles the plate by interconnecting the sections at the ribs, then bolts on the cross pieces over the ribs, then cooperatively associates the resilient members with the assembled frame, and finally inserts the frame in place and secures it by quick disconnect fastener pins into the nozzle walls.
Another nozzle plug is described by James W. Everett in U.S. Pat. No. 4,690,172, patented Sept. 1, 1987. In this application, fluid pressure is directed against the dam from the frustoconical side of the nozzle. The dam consists of two flat plate assemblies, an inner seal plate which is located in the conical portion of the nozzle, and an outer seal plate which is located further down in the nozzle and is connected to the inner seal plate by a pair of coupling rods.
Each seal plate has three segments, hinged together on the high pressure side of the plate, and an inflatable seal around the circumference of the plate to provide frictional seal between the plate and the nozzle walls. A flexible circular diaphragm covers the high pressure side of the plate. Each plate is held in its flat sealing condition by latch assemblies across the segments, on the low pressure side of the plate.
The inner seal plate has a slightly larger diameter than the outer seal plate, so that the inner seal plate is retained by its size within the frustoconical portion of the nozzle. The coupling assembly between the plates spaces them and provides for delivery of inflation fluid to the seals.
The center portion of the pressure side of the inner seal plate includes two parallel side walls interconnected at their ends by two arcuate end walls. A transverse web spanning the two side walls at the center of the section intersects a perpendicular longitudinal web which spans the arcuate end walls. Each side section of the inner seal plate has a pair of radially-extending webs along the inner surface.
Both seal plates have similar webs. The webs, by their nature cooperate with the plate to form a plurality of cavities or voids. The voids are filled with filler material such as closed cell EDPM rubber in order to provide a substantially planar top surface uniform backing support for the diaphragm.
Tabs, located only on the inner seal plate, attached to the low pressure surface of the center section, bear against the frustoconical portion of the inner surface of the nozzle to lodge the inner seal plate within the frustoconical portion, thus supporting the inner seal plate, and the outer seal plate by way of the coupling assembly, within the nozzle.
To mount the dam, in the vessel the operator unfolds and latches each seal plate to its flat sealing condition with diaphragms in place, then rotates the plates into proper alignment for the coupling assembly and joins them together parallel to each other. The coupling assembly consists of a quick-disconnect center coupler for seal pressure, and two support rods on each side of the center coupler, hinged to the inner seal plate, which are folded down for joining to the outer seal plate.
The operator then grasps the fully assembled, two plate, nozzle dam and inserts it into the nozzle, outer seal plate first. The insertion is continued until all the support tabs on the inner seal plate fully engage the frustoconical inner surface of the nozzle, whereby the nozzle seal is accurately disposed in its sealing position with the flat seal plates substantially perpendicular to the axis of the nozzle.
The inflatable seal on the outer seal plate is inflated first to prevent creep of the inner seal plate inflatable seal when it is inflated against the frustoconical portion of the nozzle.